Recovery of epsilon-caprolactone and alkane dicarboxylic acid by acid treatment and vacuum distillation

ABSTRACT

THE RECOVERY OF EPSILON CAPROLACTONE AND OF ALKANE DICARBOXYLIC ACIDS FROM MIXTURES SUCH AS ARE OBTAINED IN THE INDIVIDUAL WASHING STEPS OF THE PROCESS FOR OXIDATION OF CYCLOHEXANE WITH AIR AT ELEVATED TEMPERATURE AND AT SUPERATMOSPHERIC PRESSURE AND CONTAINING SUBSTANTIALLY MONOCARBOXYLIC ACIDS, DICARBOXYLIC ACIDS AND EPSILON-HYDROXYCAPROIC ACID (IF DESIRED AFTER PART OF THE ADIPIC ACID PRESENT HAS BEEN SEPARATED) BY REMOVING WATER (IF PRESENT) AND THE MAJOR PORTION OF THE MONOCARBOXYLIC ACIDS (IF PRESENT) BY DISTILLATION, ADDING 1 TO 30% BY WEIGHT OF PHOSPHORIC ACID OR A COMPOUND FORMING THE SAME OR BORIC ACID, HEATING THE MIXTURE OBTAINED UNDER SUBATMOSPHERIC COMPONENTS, CONDENSING THE INDIVIDUAL FRACTIONS AND RECTIFYING AND/OR CRYSTALLIZING THE FRACTIONS. DISTILLATION TEMPERATURES ABOVE 150*C. ARE AVOIDED. EPSILON CAPROLACTONE IS USEFUL FOR THE PRODUCTION OF CAPROLACTAM, A STARTING MATERIAL FOR THE PRODUCTION OF FIBERS.

United States Patent Patented Aug. 3, 1971 3,597,330 RECOVERY OFEPSILON-CAPROLACTONE AND ALKANE DKCARBOXYLIC ACHD BY ACID TREATMENT ANDVACUUM DISTHLLATION Anton Wegerich, Limbnrgerhof, Pfalz, Otto-AlfredGrosskinsky, Ludwigshafen (Rhine), Siegfried Winderl,

U.S. Cl. 203-49 4 Claims ABSTRACT UP THE DISCLOSURE The recovery ofepsilon caprolactone and of alkane dicarboxylic acids from mixtures suchas are obtained in the individual washing steps of the process foroxidation of cyclohexane with air at elevated temperature and atsuperatmospheric pressure and containing substantially monocarboxylicacids, dicarboxylic acids and epsilonhydroxycaproic acid (if desiredafter part of the adipic acid pres ent has been separated) by removingwater (if present) and the major portion of the monocarboxylic acids (ifpresent) by distillation, adding 1 to 30% by weight of phosphoric acidor a compound forming the same or boric acid, heating the mixtureobtained under subatmospheric pressure at 180 to 250 C., distilling offthe volatile components, condensing the individual fractions andrectifying and/or crystallizing the fractions. Distillation temperaturesabove 150 C. are avoided. Epsilon caprolactone is useful for theproduction of caprolactam, a starting material for the production offibers.

This invention relates to the recovery of epsilon-caprolactone andalkane dicarboxylic acids from byproducts obtained in the oxidation ofcyclohexane with air.

It is known that in the oxidation of cyclohexane with air in the liquidphase at elevated temperature and superatmospheric pressure, preferablyin the presence of metal catalysts, there are obtained, in addition tocyclohexanol and cyclohexanone, the following byproducts: acids, forexample monocarboxylic acids having one to six carbon atoms, such asformic acid, acetic acid, propionic acid, butyric acid, valeric acid andcaproic acid; dicarboxylic acids having two to six carbon atoms, such asoxalic acid, succinic acid, glutaric acid and adipic acid;hydroxycarboxylic acids, such as epsilon-hydroxy-caproic acid; aldehydicacids, keto acids and other compounds not yet identified, each having upto six carbon atoms. The major portion of these byproducts remains as aresidue in the distillation of the reaction products. They are howeverpreferably removed in the prior art methods prior to the distillation ofthe end products by Washing with water or weakly alkaline solutions, forexample alkali metal hydroxide or carbonate solutions, either after theoxidation of the cyclohexane has been completed or between theindividual stages of the oxidation, for example by the method describedin U.S. patent specification No. 2,938,- 924. Since considerable amountsof these washing solutions are obtained in industrial cyclohexaneoxidation, which is carried out on a large scale, an endeavour has beenmade to make use of the products dissolved therein. It is known that theorganic byproducts contained in the washing solutions can be oxidizedwith nitric acid, a mixture of monocarboxylic and dicarboxylic acidsbeing obtained. It is also known that adipic acid (which constitutes aconsiderable proportion of the byproducts) can be partly crystallizedout after the washing solution has been concentrated. It is said to beadvantageous for the mixture of oxidation byproducts first to beextracted with chloroform because adipic acid then crystallizes morereadily. Furthermore it is known from UK. patent specification No.997,340 that solutions which have been obtained in the alkalinehydrolysis of the byproducts formed in the oxidation of cyclohexane maybe separated into the main components by a combined extraction processwherein (after acidification) the monocarboxylic acids are first removedby means of a hydrocarbon, then the dicarboxylic acids and lactones andesters of hydroxycaproic acid are taken up by means of another organicsolvent, the solvent is distilled off, the adipic acid crystallized andseparated and finally the lactone and the ester of hydroxycaproic acidsare recovered. Finally it is known that epsilon-hydroxycaproic acid maybe obtained in monomeric and/or polymeric form from the aqueous oralkaline washing solutions obtained in the oxidation of cyclohexane withair by extraction with alcohols which are not miscible with water (cf.U.S. patent specification No. 3.277,168).

In this way however mixtures of epsilon-hydroxycaproic acid with thelactone and intermolecular esters thereof (estolides) and esters withother acids are obtained. The recovery of the main components(epsilon-hydroxycaproic acid or the lactone thereof) is thereforediflicult because the monomeric acid very readily forms esters withitself and the other acids, especially adipic acid.

It is an object of this invention to provide a process for the recoveryof epsilon-caprolactone and alkane dicarboxylic acids from mixtures suchas are obtained in the washing stages of the process for oxidizingcyclohexane with air in a relatively simple manner and with a goodyield. It is a further object of the invention to provide a process forthe recovery of the said lactone in which it is obtained in the form ofmonomeric epsiloncaprolactone and not in the form of a polymeric lactoneas in the prior art methods.

These and other objects and advantages will be better understood fromthe following detailed specification.

We have now found that epsilon-caprolactone and alkane dicarboxylicacids having up to six carbon atoms can be obtained in a relativelysimple manner and with a good yield from mixtures such as are containedin the washing solutions obtained in the oxidation of cyclohexane withair at elevated temperature and superatmospheric pressure and containingsubstantially monocarboxylic acids having up to six carbon atoms,dicarboxylic acids having up to six carbon atoms andepsilon-hydroxycaproic acid, with or without previous separation of apart of the adipic acid present, by first distilling off all orsubstantially all of any water present and if desired the major portionof the monocarboxylic acid, heating the residue at 180 to 250 C. at apressure of l to 20 mm. Hg. with an addition of l to 30% by weight,advantageously 5 to 10% by weight, with reference to the residue, ofphosphoric acid, phosphorus pentoxide and/or boric acid, thus distillingoff the volatile constituents containing episiloncaprolactone and alkanedicarboxylic acids, condensing the vapors, preferably whilefractionating them, and if desired rectifying or crystallizing theindividual fractions, distillation temperatures of more than C. beingavoided.

The starting material may be an aqueous or aqueous alkaline solutionwhich has been obtained by washing oxidation mixtures such as areobtained in the oxidation of cyclohexane with air in the liquid phase atelevated temperature, for example at 130 to C., and at superatmosphericpressure, for example at 2 to 50 atmospheres preferably in the presenceof oxidation catalysts, particularly cobalt compounds, with water oraqueous-alkaline solutions, for example up to 30 wt. percent alkalihydroxide solutions either after the oxidation of cyclohexane or betweenthe individual oxidation stages. They contain from about 30 to 50% byweight of organic substances. The percentage contents of the mostimportant components are about within the following limits, in each casewith reference to the total amount of organic substances:

(a) monocarboxylic acids: acetic acid, propionic acid, butyric acid, upto about 10% by weight valeric acid, caproic acid, up to about 20% byweight,

(b) dicarboxylic acids: succinic acid, glutaric acid, about 5 to 10% byweight, adipic acid up to about 30% by weight,

() hydroxycaproic acid and other carboxylic acids, omega-hydroxy caproicacid (also as lactone), about 25 to 40% by weight.

When starting from alkaline solutions, these are first acidified,preferably with a mineral acid such as hydrochloric acid or sulfuricacid and then the inorganic salts formed are separated, for example byextracting the free organic acids (for example with ketones such asacetone, methyl ethyl ketone or cyclohexanone, or with aromatichydrocarbons such as toluene or xylene) before or after the water isseparated, and the extracted acid mixture is used as starting materialfor the process. Such mixtures in general contain only very little waterso that separate dehydration can often be dispensed with.

In addition to the said acids, there is a number of other byproductsformed in the oxidation of cyclohexane, some of which are not preciselyknown, contained in the said Washing liquids. It is also possiblehowever to use mixtures of this type from which some of the adipic acidhas already been separated, for example after concentration andcrystallization of some of the adipic acid. These solutions (which forthe sake of simplicity are also referred to as washing solutions)contain up to 75% by weight of organic substances of the same types asspecified above, the percentage content varying according to the extentof the separation of adipic acid. Finally it is possible to use anymixtures of monocarboxylic acids, dicarboxylic acids andhydroxycarboxylic acids whose composition is similar to that of the saidwashing solutions.

Monomeric epsilon-caprolactone is obtained by the process according tothis invention from epsilon-hydroxycaproic acid or polymers or estersthereof. More or less of various byproducts, for example hexenoic acid,methyl-fi-valerolactone or 'y -ethyl-' -butyrolactone, are obtaineddepending on the conditions used.

The water is first substantially evaporated, for example down to acontent of 5 to by weight but preferably practically completely. Ingeneral not more than 5% by weight of water remains in the residue. Thewater is distilled off advantageously first at atmospheric pressure andthen in a second stage at subatmospheric pressure, for example down to 5mm. Hg. Volatile carboxylic acids, for example acetic acid andhomologues up to caproic acid, and also remainders of cyclohexanoland/or cyclohexanone evaporate together with the water. Duringevaporation at subatmospheric pressure, small amounts ofepsilon-caprolactone and also keto acids (mainly laevulinic acid) andsuccinic anhydride also pass over. The water is distilled offpractically completely for example by heating at subatmosphericpressure, for example up to about 220 C. at 20 mm. Hg. Duringevaporation, epsilon-hydroxycaproic acid is converted, with eliminationof water, into polymers and into ester compounds with the dicarboxylicacids. The monocarboxylic acids can readily be distilled off from thishigh boiling point residue. Purification of the lactone to be recoveredis thus simplified. Phosphoric acid, phosphorus pentoxide, boric acid orboric anhydride is added to the residue obtained (with or withoutcooling, for example to a temperature below 4 200 C.) advantageously inan amount of 3 to 20%, particularly 5 to 10%, by weight with referenceto the residue. It is advantageous for the phosphoric acid or boric acidused to be substantially anhydrous, for example an or more highlyconcentrated solution.

After these acids have been added, the residue is further heated at apressure of l to 20 mm. Hg to 180 to 250 C. A mixture of lactones anddicarboxylic acids distills over. It is condensed, preferably withfractionation, and then if desired pure lactone is recovered from thedistillate by rectification and if desired pure dicarboxylic acids areobtained advantageously by crystallization. Fractional condensation isadvantageously carried out in the first stage at about to 170 C. and ina second stage at room temperature. In the first fraction separated atthe higher temperature, a mixture of adipic acid, glutaric acid andsmall amounts of lactone (about 10%) are obtained and in the secondfraction separated at room temperature mainly lactones are obtained withsmall amounts of dicarboxylic acids or anhydrides.

The effects of phosphoric acid and boric acid are not entirelyequivalent. When using boric acid mainly epsiloncaprolactone is obtainedin addition to the dicarboxylic acids and only a small amount ofhexenoic acid as a byproduct. When using phosphoric acid, there isformed not only epsilon-caprolactone but also methylvalerolactone andethylbutyrolactone, which are formed by secondary reactions, and thereis a general rule that the amount of byproducts increases when higherpressure and temperature are used and when a larger amount of phosphoricacid is added. The presence of steam also increases the proportion ofbyproducts. Practically only ethylbutyrolactone is obtained in a steamdistillation at atmospheric pressure. In continuous operation it is advantageous to remove continuously from the bottoms (which is heated at200 to 250 C.) a portion, for example 20 to 30% by weight, of the amountsupplied and to keep the proportion of phosphoric acid or boric acid inthe bottoms at a constant value, for example at 5 to 10% by weight ofthe bottoms, by adding the said acid.

The distillate or the fractions of the distillate are as a rule notuniform in composition. They may be separated into the individualcomponents however by simple crystallization and rectification.Appreciable changes, for example by fresh esterification with thecarboxylic acids or by the formation of estolides, do not take placewhen tem peratures below C. are maintained during the distillation. Itis advantageous to convert the anhydrides with a small amount of waterat 60 to 80 C. into the high boiling point acids and to separate thelactones from these by rapid distillation. The lactones are separated bysubsequent rectification, advantageously at pressures of less than 10mm. Hg. The dicarboxylic acid fraction is advantageously purified byfractional crystallization from water or solvents such as xylene; adipicacid is recovered as a crystallization product; glutaric acid, lactoneand impurities are contained in the mother liquor.

The epsilon-caprolactone obtained after rectification is equivalent toepsilon-caprolactone obtained by other methods, for example fromcyclohexanone by the Bayer- Villiger reaction, is stable and can be usedin the same way as the said other product. It may be further purified ifnecessary by crystallization from the melt (melting point 5 C.).

The phosphoric acid or boric acid in the distillation residue can easilybe recovered because these residues are substantially soluble whenboiled, advantageously at super atmospheric pressure and elevatedtemperature, for example at 5 atmospheres gauge and 150 C., in once totwice the amount of aqueous starting solution, and can be used asadditions after insoluble material has been filtered ofi and waterremoved. The small amount of organic compounds which is recycled in thisway is not troublesome.

The invention is illustrated by the following example.

EXAMPLE When Washing with water an oxidation mixture obtained byoxidation of cyclohexane with air, a solution is obtained containing 30%by Weight of organic constituents which comprise 38% by weight ofepsilon-hydroxycaproic acid (present in monomeric form, in polymericform or as hydroxycaprolactone), 2% by weight of monocarboxylic acids(acetic acid, caproic acid and valeric acid), 40% by weight ofdicarboxylic acids (adipic acid, glutaric acid and succinic acid), 3% byweight of keto acids, mainly laevulinic acid, and 17% by weight ofunknown condensation products. This aqueous solution is firstsubstantially freed from water at atmospheric pressure by heating to 150C., acetic acid and traces of cyclohexanol and cyclohexanone distillingoif with the steam. 1,000 g. of the concentrate thus obtained is heatedto 220 C. while gradually decreasing the pressure. 83 g. of an aqueousdistillate distils over up to 200 C. and at a pressure of down to 20 mm.Hg. In the range up to 220 C. and a pressure decrease to 5 mm. Hg,another 69 g. of organic products distil over. They comprise 27.8% byweight of epsilonhydroxycaprolactone, 26.5% by Weight of keto acids(mainly laevulinic acid) and succinic anhydride with a small amount ofadipic acid.

by weight of 85% phosphoric acid is added to the practically anhydrousresidue and then the solution is distilled at 5 mm. Hg and 214 C. Thevapor is first passed through a cooler cooled with boiling xylene wheremainly high boiling point dicarboxylic acids are condensed. In a secondcooler charged with cooling water at room temperature, a fractioncondenses consisting mainly of lactones. From 3,000 g. of thepractically anhydrous residue there are obtained by the addition of 300g. of 85% phosphoric acid the following fractions: 1,050 g. of lactonefraction (containing 75% of lactones and of hexenoic acid; the remainderis glutaric anhydride, glutaric acid and adipic acid); 790 g. of adicarboxylic acid fraction (adipic acid and glutaric acid); 270 g. ofreadily volatile constituents, including cyclohexene, cyclopentanone andwater (separated by strong cooling upstream of the vacuum pump. Any COformed is sucked off by the vacuum pump).

A residue of 1,130 g. remains in the evaporator. It is liquid. If it ismixed while heating to 150 C. at 5 atmospheres gauge with an equalamount of aqueous Washing solution such as has been used as startingmaterial the major portion dissolved and 90% of the phosphoric acid isfound in this solution. After this solution has been dehydrated it isused again as an addition of phosphoric acid. The insoluble residue (320g.) becomes solid at temperatures of about +10 C., is soluble in acetonebut cannot be hydrolized with caustic soda solution. 50 g. of water isadded to the lactone fraction (1,050 g.) and the whole heated to C. inorder to convert the anhydrides into acids. The lactones are separatedfrom the high boiling point dicarboxylic acids by rapid distillation at10 mm. Hg and temperatures of up to C. 525 g. of epsiloncaprolactone,210 g. of -ethyLy-butyrolactone and 53 g. of fi-methyl-fi-valerolactoneare recovered in pure form from this lactone fraction by rectification.

The dicarboxylic acid fraction is dissolved in an equal weight of hotXylene. Adipic acid crystallizes out upon cooling. 70% of thedicarboxylic acid fraction is recovered as pure adipic acid. Smallamounts of adipic acid, glutaric acid, lactone and impurities remain inthe mother liquor.

We claim:

1. An improved process for recovering epsilon-caprolactone and alkanedicarboxylic acids from solutions which are formed by washing mixturesobtained in the oxidation of cyclohexane with air at elevatedtemperatures and at superatmospheric pressures with water or weaklyalkaline solutions, said formed solutions containing monocarboxylicacids, dicarboxylic acids, and epsilon-hydroxycaproic acid, saidimproved process comprising: distilling off substantially all of anywater present in the washing solution together with the major part ofany monocarboxylic acids that are present; heating the residue and from1 to 30% by weight, with reference to the residue, of at least onemember of the group consisting of phosphoric acid, phosphorus pentoxide,and boric acid, to a temperature of from to 250 C. at a pressure of l to20 mm. Hg, whereby the volatile constituents in the residue aredistilled off; and thereafter fractionally condensing the vapor toobtain a fraction containing mainly epsilon-caprolactone and a fractioncontaining mainly alkane dicarboxylic acids.

2. A process as claimed in claim 1, wherein the fraction containingmainly epsilon-caprolactone is subsequently rectified at a temperatureup to 15 0 C.

3. A process as claimed in claim 1, wherein the fraction containingmainly alkane dicarboxylic acids is crystallized.

4. A process as claimed in claim 1 wherein the amount of acid added tosaid residue is from 5 to 10% by weight based on the weight of theresidue.

References Cited UNITED STATES PATENTS 3,189,619 6/1965 Aldridge et al.260-343 3,227,730 1/1966 Goldsmith et al. 260-343 3,277,168 10/1966Koenig 260-343X WILBUR L. BASCOMB, JR., Primary Examiner U.S. Cl. X.R.

